1. Field of the Invention
The present invention relates to novel thermoplastic block polymers and methods of making same; and high-energy compositions, such as propellants, explosives, gasifiers, and the like, containing such block polymers therein as binders, which are chemically uncured.
2. Description of the Prior Art
Solid high-energy compositions, such as propellants, explosives, gasifiers, or the liked comprise solid particulates, such as fuel particulates and/or oxidizer particulates, dispersed and immobilized throughout a binder matrix comprising an elastomeric polymer.
Conventional solid composite propellant binders utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents, as typically disclosed in U.S. Pat. No. 4,361,526 and European Patent No. 266,973. As outlined in detail in the referenced U.S. patent there are important disadvantages to using cross-linked elastomers as binders. Cross-linked elastomers must be cast within a short period of time after addition of the curative, which time period is known as the xe2x80x9cpot lifexe2x80x9d. Disposal of a cast, cross-linked propellant composition is difficult, except by burning, which poses environmental problems. Furthermore, current state-of-the art propellant compositions have serious problems that include, but are not limited to use of nonenergetic binder, high end-of mix viscosities, toxic isocyanate curatives, thermally labile urethane linkages, and vulnerability to unscheduled detonation.
Although advantages of thermoplastic elastomers relative to cured elastomers for use in high-energy compositions are appreciated by those skilled in the art, particularly for use in rocket motors, there are reasons why one might not rush to substitute thermoplastic elastomers for cured elastomers in specific applications. Rocket motors are expensive to design, test and produce. Typically, a rocket motor is developed to use a particular propellant composition which has certain mechanical and burn characteristics. If a rocket motor design is successful, there is reluctance to make changes, and particularly to make changes with respect to the propellant composition. Furthermore, designers of rocket motors have a great deal of experience with propellant compositions having cast-cured propellant compositions and are less familiar with the characteristics of propellant compositions having thermoplastic binders.
It is believed that high-energy compositions, particularly solid rocket motor propellants, having thermoplastic binders will achieve greater acceptance if they closely approximate the mechanical and burn characteristics of currently used cast-cured propellant compositions. The present invention is particularly directed to propellant compositions which attempt to approximate the mechanical and burn characteristic of poly(butadiene) based cured propellant compositions.
U.S. Pat. Nos. 3,585,9257 and 4,360,643 and European Patent No. 235,741, in general disclose various block polymers, e.g. of butadiene and lactones, and methods of making same, but nowhere suggest using any such block polymer systems as the binder component in high energy composition such as a propellant for a rocket motor.
Polymers derived in the prior art from branched claim lactones such as 6-hydroxy dodecanoic acid lactone (epsilon or xcex5-laurolactone) and 4-hydroxy dodecanoic acid lactone (gamma or xcex3-laurolactone), and unsubstituted lactones such as 6-hydroxy hexanoic acid lactone (epsilon or xcex5-caprolactone), are not acceptable for the disclosed utility according to the invention because the melting points of these polymers are below that minimally useful, i.e. less than 70xc2x0 C.
The methods of making the block polymers as disclosed in these three references are materially different from the methods according to the present invention (particularly as set forth in Examples 1-3 to follow). The lithium, sodium and potassium based methods described therein are not applicable to the preparation of low polydispersity, high molecular weight 12-hydroxy dodecanoic acid lactone (hereinafter xcex-laurolactone) polymers in high conversion. None utilize a zinc counter ion wherein diethyl zinc reacts with the hydroxyl end group of the butadiene polymer to form an intermediate zinc alkoxide-tipped polybutadiene which functions as the active initiating species, as in the present invention. Similarly none link previously prepared difunctional hydroxyl-terminated polybutadiene blocks with monofunctional poly (xcex-laurolactone) blocks using a diisocyanate and a catalyst, as in the present invention.
An earlier parallel effort of applicant with polyoxetanes to form (AB)n type materials using a similar method is the subject of U.S. Pat. No. 4,806,613. This patented method for oxetanes, wherein each of the two blocks is separately allowed to react with a difunctional isocyanate to form isocyanate capped blocks which are then linked with butanediol, is quite different from the present invention.
The object of the present invention is to provide novel thermoplastic elastomers and methods of making same; and chemically uncured high-energy compositions, particularly rocket motor propellants, formed therefrom.
The thermoplastic elastomers are block polymers in which the polymer molecules comprise at least one poly(butadiene) block which is amorphous in the range of ambient temperatures and at least one pair of poly(lactone) blocks, which are crystalline at temperatures up to about 70xc2x0 C. flanking the polybutadiene block. The lactones from which the poly(lactone) blocks are formed contain between 8 and 18 carbon atoms in the lactone ring with no carbon or heteroatom substituents other than hydrogen on the ring.
The poly(lactone) blocks preferably contain between 10 and 18 carbon atoms in the lactone ring, and most preferably between 10 and 12 carbon atoms. The preferred lactones are 10-hydroxy dodecanoic acid lactone and 12-hydroxy dodecanoic acid lactone (lambda or xcex-laurolactone), the latter is most preferred. The derived poly(lactones) are, therefore, linear polyesters without carbon or heteroatom side chains.
According to the present invention two methods of preparing ABA triblock polymers are disclosed. In the first, a zinc counter ion is utilized wherein diethyl zinc reacts with the hydroxyl end group of the hydroxy terminated butadiene polymer to form an intermediate zinc alkoxide-tipped polybutadiene which functions as the active initiating species. The laurolactone polymer grows off the hydroxyl end groups of the butadiene to form the desired ABA block copolymer, producing low polydispersity, high molecular weight xcex-laurolactone polymers in high conversion, which characteristics are critically important for the destined end application/utility, i.e. as a binder for high energy propellant compositions. In the second, previously prepared difunctional hydroxyl-terminated polybutadiene blocks are linked with monofunctional poly (xcex-laurolactone) blocks using a diisocyanate and a non-metal based catalyst. This technique allows the use of a xe2x80x9cbuilding blockxe2x80x9d approach to the preparation of large numbers of distinct block copolymers from a smaller number of xe2x80x9cbuilding blocksxe2x80x9d and also circumvents the use of a metal based catalyst in the block polymerization scheme. The poly (xcex-laurolactone) is prepared from xcex-laurolactone, diethyl zinc and a suitable mono-functional alcohol such as benzyl alcohol.
Also according to the present invention a method of preparing a different class of thermoplastic elastomeric materials with an (AB)n structure (as opposed to the ABA structures resulting from the two methods aforementioned) is disclosed wherein butadiene and xcex-laurolactone are linked or polymerized. By this method the direct linking of a hydroxyl-terminated poly(butadiene) of functionality 2.0 with a poly(xcex-laurolactone) also with a functionality of 2.0 with a difunctional isocyanate occurs. This somewhat different technique parallels an earlier effort of applicant with polyoxetanes to form (AB)n type materials using a similar method which is the subject of U.S. Pat. No. 4,806,613, as aforementioned.
Chemically uncured, high-energy compositions made in accordance with the invention comprised the aforementioned thermoplastic elastomers and high-energy particulates, such as oxidizer particulates and/or fuel materials, e.g. particulates. The uncured high-energy composition may also optionally include a plasticizer, preferably mineral oil, if used.
Other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and appended claims.
In accordance with the invention, thermoplastic block copolymers are produced in which at least one amorphous poly(butadiene) block is flanked by at least one pair of crystalline poly(lactone) blocks. Such block polymers are suitable for use as binders for high-energy compositions, such as gasifiers, explosives, and particularly propellants. The chemically uncured thermoplastic binder system formed from the block polymers have mechanical and burn properties similar to those using cross-linked (cured) poly(butadiene) as the binder. The poly(butadiene) blocks are amorphous and elastomeric at ambient temperatures, i.e. at 20-25xc2x0 C. and are preferably amorphous at low temperatures, e.g., down to 40xc2x0 C. and even down to xe2x88x9260xc2x0 C. and below. The poly(lactone) blocks, on the other hand, are crystalline at ambient temperatures and remain crystalline to at least 70xc2x0 C. The melting temperature of the poly(lactone) is such that at temperatures above 70xc2x0 C. and below 120xc2x0 C. (preferably below 100xc2x0 C.) the block copolymer is processable; that is, meltable, easily castable when melted, and thermally cyclable. It is preferred that this temperature be above 70xc2x0 C. and below 100xc2x0 C., and most preferably at a temperature of about 80xc2x0 C. The invention includes block polymers of the (AB)n (n=2-40) type, the ABA type and AnB (n=3,4,5) star polymer type, where B is the poly(butadiene) blocks(s) and A blocks are poly(lactone). The lactone monomers from which the poly(lactone) blocks are formed have between 8 and 18 carbon atoms in the lactone ring with no carbon or heteroatom side chains other than hydrogen. Preferably the lactone monomers contain between 10 and 18, and most preferably between 10 and 12, carbon atoms in the lactone ring. Suitable lactones include 10-hydroxy decanoic acid lactone, and xcex-laurolactone, the latter being preferred. Mixtures of these two lactones are also suitable.
The preferred method of forming the block copolymers of the invention is to separately form the poly(butadiene) blocks and the poly(lactone) blocks and then join the blocks. For example, hydroxyl-terminated poly(butadiene) (HTPB) may be joined to hydroxyl-terminated poly(lactone) blocks with a diisocyanate. Alternatively, as described in U.S. Pat. No. 4,806,613 issued Feb. 21, 1989 to Robert Wardle, the teachings of which are incorporated herein by reference, the polymer blocks might each be capped with a diisocyanate and subsequently connected with a short-chain diol. A less preferred method of forming block polymers in accordance with the invention is to provide a HTPB block and polymerizing a lactone from the hydroxyl termini of the HTPB block.
The poly(butadiene) blocks(s) provides the elasticity of the thermoplastic elastomeric copolymer. The poly(butadiene) block is selected to be amorphous throughout the temperature range to which a high-energy composition is expected to be exposed. For various propellant applications this may be down to about xe2x88x9240xc2x0 C. or even xe2x88x9260xc2x0 C. and below. On the other hand, the poly(lactone) blocks, which provide rigidity to the thermoplastic elastomer, are expected to remain crystalline throughout the temperature range to which the high-energy composition might be proposed. Generally, it is considered that propellants may be exposed to temperatures up to 60xc2x0 C., and herein, poly(lactone) blocks are selected so that the thermoplastic elastomeric block copolymer does not become mobile until at least about 70xc2x0 C. Because of the presence of high-energy particulates, particularly high-energy oxidizer particulates, the block copolymer should be processable below about 120xc2x0 C. preferably below about 100xc2x0 C. and most preferably at about 80xc2x0 C. Suitable lactone monomers for forming block copolymers which are processable at the appropriate temperature ranges have between 8 and 18 carbon atoms in the lactone ring with no carbon or heteroatom side chains other than hydrogen. The lactone preferably contains between 10 and 18, and most preferably between 10 and 12, carbon atoms in the lactone ring. Specifically the lactones most suitable for forming the poly(lactone) blocks in accordance with the present invention include 10-hydroxy decanoic acid lactone (structure I), lambda or xcex-laurolactone (Structure II) and mixtures thereof. The most preferred poly(lactone) blocks are homopolymers of xcex-laurolactone. These two preferred lactones are diagramed as follows: 
An ABA type polymer derived from the most preferred structure II xcex-laurolactone block with butadiene derived block is diagramed as follows: 
Thus it can be seen that the derived poly(lactones) in accordance with the invention are linear polyesters without carbon or heteroatom side chains. As earlier indicated, polymers derived from branched chain lactones such as 6-hydroxy dodecanoic acid lactone (xcex5-laurolactone), 6-hydroxy hexanoic acid lactone (xcex5-caprolactone) and 4-hydroxy dodecanoic acid lactone (xcex-laurolactone), as well as unsubstituted lactones such as 6-hydroxy hexanoic acid lactone (epsilon or xcex5-caprolactone) are not acceptable.
To provide the requisite elasticity, relatively long chain poly(butadiene) blocks are incorporated in the block polymers. Poly (butadiene) blocks useful in accordance with the invention each have a molecular weight (Mw) in the range of between about 10,000 and about 100,000 and preferably between about 20,000 and about 50,000. The polydispersity of the poly(butadiene) blocks is preferably below about 1.6. The poly(lactone) blocks provide structure to the block copolymer, but do not contribute to elasticity. Accordingly, the total weight of the poly(lactone) blocks is less than that of the poly(butadiene) blocks, typically between about 10 and about 30 wt. percent of the poly(butadiene) blocks.
The type of polymer which forms depends upon the functionality of the block and the molar ratios of the blocks and linking compounds. The poly(lactones) can be mono or difunctional; typically the poly(butadiene) is also difunctional. To form an ABA polymer, a monofunctional poly(lactone), linker, and poly(butadiene) are reacted in approximately a 2:2:1 molar ratio. An (AB)n block copolymer will formed if a difunctional poly(lactone) is used and the molar ratio is approximately 1:(0.6-0.9):1. To form a AnB polymer, a multi-armed poly(butadiene) must be employed. ABA polymers may also be produced by polymerizing the poly(lactone) block from the ends of a difunctional HTPB.
To form high-energy compositions, particularly solid rocket propellants, which are generally a two phase system of binder and solids, the block polymer is mixed at processing temperature with solids, including fuel material particulates, e.g., aluminum, and oxidizer particulates, e.g., ammonium perchlorate (AP) cyclotetramethylene tetranitramine (HMX) and cyclotrimethylene trinitramine (RDX); and plasticizers. Then, the molten composition is cast, e.g., into a rocket motor casing, and the composition is allowed to cool. And in no case is chemical curing agents used.
Such high energy propellant formulations typically contain between about 70 to 91% solids, including oxidizer particulates and fuel material particulates, with the balance, i.e. about 9 to 30%, being a binder system including the block polymer and optionally plasticizer (all percentages by weight). The plasticizer can be dioctyladipate, mineral oil, silicon oil, xcex2-Pinene, ACTIPOL E6 (registered trademark of AMOCO, Naperville, Ill.), KRYTOX (registered trademark of Dupont Co., Wilmington, Del.), ARNEEL (registered trademark of Akzo, Chicago, Ill.), or a mineral oil and poly (10-hydroxy dodecanoic acid) homopolymer mixture. Of these plasticizers, mineral oil or mineral oil in admixture with poly(10-hydroxy dodecanoic acid) homopolymer are preferred, with mineral oil most preferred. The preferred formulation contains about 85-91% solids and about 9-15% binder. The most preferred propellant mixture contains about 85% solids and about 15% binder, as indicated in examples 4 and 5 to follow. The solids are normally ammonium perchlorate and aluminum with the aluminum making up from 10-24% of the total propellant, preferably 13-18%, with ammonium perchlorate making up the balance of the solids. In examples 4 and 5, the aluminum is either 16 or 17%. of the total propellant weight. The solids are typically finely ground to allow good mixing and bonding with the binder. The aluminum used was 23xcexc average diameter with the ammonium perchlorate being a 3.06:1 ratio of 200xcexc and 20xcexc average particle sizes. These particle sizes and ratios are typical for a propellant of this type, although an infinite number of combinations would be useful as long as a reasonable distribution is maintained (i.e. some larger sized particles and some smaller sized particles). This mixture of particle sizes is needed to optimize particle packing for a good propellant density and mechanical properties and still have a low enough viscosity to process. The large particle sizes help with processing and the smaller with packing and properties. Plasticizer for the block polymer is typically employed at a ratio relative to thermoplastic copolymer (P1/Po) of between about 0.2 (1:5) and about 4.0 (4:1). The viscosity of the propellant at processing temperatures of 80-85xc2x0 C. at the end of the mix cycles must be below 60 Kilopoise and preferably below 20 kilopoise.
The invention will now be described in greater detail by way of specific examples. Examples 1 and 3 relate to two different methods utilized for the preparation of ABA triblock materials while Example 2 relates to the synthesis of an (AB)n material. Examples 4 and 5 are illustrative of specific propellant formulations. It is significant to note that at no stage in the processing is the binder system chemically cured.